The present invention relates to an optical waveguide device, and more particularly to an optical waveguide device having an transparent waveguide member.
In, for example an optical communication system, the optical waveguide device is one of the most important members comprising the system, because, the optical waveguide device is used to make an optical switch, an optical modulator, an optical isolator or an optical circulator. Especially, the present invention refers to a device comprising an electro-optic crystal, or a device comprising a magneto-optic crystal.
As is widely known, the optical waveguide device made of the electro-optic crystal or the magneto-optic crystal is utilized for switching a plane of polarization so as to switch the optical path as a non-mechanical transducer. Generally, a mechanical switch, having the same function for switching light paths, produces a disadvantage in that it is difficult to increase the reliability and also achieve a high speed switching operation. However, the above mentioned non-mechanical switch does not produce such disadvantages.
The optical waveguide device made of the electro-optic crystal can change its plane of polarization by 90.degree. or 0.degree. depending upon whether or not an external electric field is applied thereto. Similarly, the optical waveguide device, made of the magneto-optic crystal can change its plane of polarization by +45.degree. or -45.degree. when an external magnetic field is applied in a forward direction or a reverse direction. Regarding the electro-optic crystal, it is preferable to reduce the thickness thereof to be as small as possible so as to obtain a high electric field strength created therein. Similarly, regarding the magneto-optic crystal, it is preferable to reduce the thickness thereof to be as small as possible so as to decrease the demagnetizing field induced therein and, accordingly, obtain a high magnetic field strength created therein. However, it is very important to notice that when the thickness of the transducing crystal is reduced to be as small as possible, the function of changing the plane of polarization can no longer be maintained with a high degree of accuracy. That is, for example, deleterious crosstalk is generated in the crystal. Briefly the reason for this is as follows. When the crystal is made small in thickness, for example 50 .mu.m, light, to be propagated in the crystal cannot pass therethrough without abutting against the walls thereof. Thus, the light passes through the crystal while reflecting off the wall repeatedly. In this case, a phase shift induced by the occurrence of each reflection of a reflected P polarized light component is not the same as a phase shift of a reflected S polarized light component. As is known by persons skilled in the art, the P polarized light component is a parallel component with respect to a plane along which the light runs in an electric field direction. However, the S polarized light component is perpendicular to the above mentioned plane. Since, as mentioned above, the amount of the phase shift of the reflected P polarized light component is not the same as that of the reflected S polarized light component, the light to be propagated in the crystal cannot pass therethrough without changing its polarization state, which is defined by the combination of both the P and S polarized light components. In such a case, the light output from the crystal cannot be accurately polarized and, thus, the previously mentioned deleterious crosstalk is produced thereby.